Driver side airbag modules, which include an airbag cushion and an airbag module cover, are normally positioned within a hub of a steering wheel of an automobile. This happens to be the same area which conventionally includes the horn switch. Accordingly, the airbag module cover must additionally serve to actuate the horn switch, preferably over a substantial area.
A number of horn switch designs have included a membrane type switch. Membrane type switches conventionally comprise two very thin sheets having conductive coatings which are normally separated by thin spacers. Pressure on the switch pushes the conductive surfaces together to close a circuit and actuate the horn. Pressure is applied to the horn switch by the airbag module cover in the hub of the steering wheel, which is depressed by the driver.
The introduction of the airbag module at the steering wheel hub complicated the design of the horn switch and the airbag module cover. For example, the cover should not require an unreasonable degree of depression or an unreasonable amount of applied pressure to actuate the horn switch, the horn switch should not be susceptible to inadvertent actuation, and the horn switch must not become unattached from the airbag module cover upon inflation of the airbag.
In addition, the airbag modules including the membrane horn switch are required to pass certain tests after and during which the module is subjected to environmental conditioning. This conditioning includes subjecting the module to thermal, humidity and vibration cycling that in most cases exceeds the average operational life of an automobile. For example, the temperature and humidity may range from 90.degree. C. and 95% relative humidity to -40.degree. C. These extreme conditions also make the design of a membrane horn switch very difficult.
In some earlier prior art membrane horn switch and airbag module cover configurations, the switch was compressed against the folded airbag cushion itself located behind the airbag module cover. These configurations required a large degree of airbag module cover depression and a large amount of force to activate the switch.
Some later prior art membrane horn switch and airbag module cover configurations tried to overcome these problems by securing the switch directly to the airbag module cover or tightly sandwiching the switch between the cover and an attached switch backing plate. Yet, these combinations made the switches susceptible to inadvertent actuation due to thermal contraction or expansion of the airbag module cover at extreme temperatures. The amount of pressure required to activate the horn switch also varied substantially as a function of ambient temperature, which affects the flexibility of the cover and backing plate. Some prior art membrane switches also included localized raised areas located on the front of the switches. The raised areas faced the module cover and acted as force concentrators that decreased the amount of applied force needed to actuate the switch.
In summary, it would be useful to provide an airbag module cover having a horn switch that does not require an unreasonable degree of cover depression or an unreasonable amount of applied pressure for actuation, and is not susceptible to inadvertent actuation resulting from thermal expansion or contraction of the cover.